Devices for converting thermal directly to electric energy have been extensively investigated. The most commonly utilized devices have been thermionic converters working from high temperature sources and silicon cells utilizing the input thermal energy of solar radiation. Such devices have limited operating temperature ranges. In addition, the efficiencies for the direct conversion of solar energy to electricity that are required to be competitive with conventionally generated electricity in many U.S. markets over the next few decades have not yet been achieved. These required efficiencies range from the lowest efficiency requirement of 10% module efficiency for flat plate modules to the highest efficiency requirement of 25% module efficiency for concentrated solar energy. These required efficiencies are reported by E. A. DeMeo et al in Proceedings of the 21st Institute of Electrical and Electronics Engineers Photovoltaic Specialists Conference--1990, Kissimmee, May 21-25, 1990 (Institute of Electrical and Electronics Engineers, New York, 1990), pp. 16-23.
A reversible thermoelectric converter having a high operating efficiency is disclosed in U.S. Pat. No. 4,004,210 issued Jan. 18, 1977 to Yater. The disclosed device comprises a first layer of microcircuit modules for converting thermal energy into electric voltage fluctuations, a second layer of microcircuit modules for receiving the electric voltage fluctuations and a third layer between the first and second layers. The third layer is a thermal barrier, such as a vacuum. Electric voltage fluctuations are capacitively coupled from the first layer across the thermal barrier to the second layers. The first and second layers operate at different temperatures. The microcircuit modules can be Schottky barrier diodes or quantum or tunnel diodes. The reversible thermoelectric converter disclosed in U.S. Pat. No. 4,004,210 has the potential to achieve efficiencies as high as 90%. The thermal barrier transmits electric voltage fluctuations and prevents cooling, radiation losses and lead conduction losses.
The theoretical basis for the operation of reversible thermoelectric converters is described by J. C. Yater in Physical Review A, August 1979, pages 623-627, J. C. Yater in Physical Review A, July 1982, pages 522-538 and by J. C. Yater in Solar Cells, Vol. 10, August 1983, pages 237-255. These articles describe efficiencies up to 99% of the Carnot cycle efficiency and describe physically realizable diode designs, including thin film, quantum effect and thermionic, that can enable high power output and high efficiency to be achieved. All of the previously disclosed reversible thermoelectric converters known to applicants include a separate thermal barrier between the hot and cold layers of the circuit.
Quantum size effects in thin metal films is described by R. C. Jaklevic et al in Physical Review B, Vol. 12, No. 10, Nov. 15, 1975, pages 4146-4160. Devices involving quantum effects are described by F. Capasso et al in "Quantum Electron Devices", Physics Today, February 1990, pages 74-82. A typical quantum well diode includes a thin dielectric layer between two thin metal layers. Electrons in the metal layers are quantized into discrete energy levels in a direction perpendicular to the metal surface. The dielectric layer forms a potential barrier.
It is a general object of the present invention to provide improved reversible thermoelectric converters.
It is another object of the present invention to provide an improved reversible thermoelectric converter including a thin film quantum well diode for directly converting thermal energy of hot electrons to electric energy.
It is a further object of the present invention to provide reversible thermoelectric converters with improved efficiency of conversion of solar energy to electrical energy.
It is yet another object of the present invention to provide hot electrons for the reversible thermoelectric converter from different heat sources including fossil, nuclear and geothermal heat sources.
It is a further object of the present invention to provide efficient operation of the reversible thermoelectric converter at low temperatures.